This invention relates to a process for preparing high purity, stoichiometric aluminum silicate (mullite).
Mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) has long been known in the ceramic and refractory industries. Mullite is the most stable compound in the Al.sub.2 O.sub.3 -SiO.sub.2 system. Consequently, it occurs as a main constituent in a large number of ceramic products which are fabricated from aluminosilicate materials. Considerable amounts of mullite are used to produce refractory bodies designed to withstand high temperatures. Its relatively low thermal coefficient of expansion makes such refractories more resistant to thermal stress in constrast to similar bodies prepared from aluminum oxide materials.
Mullite possesses a dielectric constant of approximately 5 to 6 and therefore presents a very attractive electrical characteristic as integrated circuit technology continues advancing to higher speed circuit devices. Moreover, mullite's low thermal coefficient of expansion offers an excellent match to large silicon integrated circuit chips or glass layers which may be placed on substrates. Although mullite has been mentioned for use as multi-layer electronic substrates for integrated circuit devices, high purity, dense substrates are not known to exist.
Commercially available mullite always contains significant amounts of impurities such as silica, iron oxide, and titania. These impurities influence the physical, electrical and chemical properties of the mullite, which in turn affect the ceramic compositions of which mullite may be embodied in.
The most common technique for mullite fabrication involves the heating of clays, feldspars, kyanites, etc. to a temperature in excess of 1300.degree. C. The degree of completeness of the reaction is dependent on temperature and the time the sample is held at temperature. The higher the reaction temperature, the less the dwell time at temperature. During heating, the clay structure breaks down to form mullite and an amorphous silica phase. This silica glass is very viscous and can either crystallize to a crystalline silica phase or it can react with excess alumina that may have been added to the initial raw material mixture. This reaction will also yield mullite. Again, the degree of completion of this reaction is dependent on temperature and sample time at temperature.
If alumina and clay are mixed in the proper properties, production of a 100% mullite body is feasible. However, as the chemical reaction sequence has a volume change associated with it, the fabrication of 100% mullite articles by this technique to rigid dimensional specifications becomes very difficult, if not impossible. Therefore, the common method to circumvent this problem is to pre-react a portion of the material to 100% mullite and then grind this material and add it to a mixture of the initial raw material mixture. The ceramic industry term for any pre-reacted material is grog. This mixture of grog and initial raw material mixture often called binder is then fabricated into the desired shape and sintered at a high temperature to convert the raw material mixture to mullite and drive the sintering reaction to a satisfactory end-point. Variations of this technique are possible.
Mullite can also be fabricated by processes that do not use any glass phase point in the process. This solid state reaction technique makes use of the fact that the equilibrium Al.sub.2 O.sub.3 -SiO.sub.2 phase diagram predicts that if Al.sub.2 O.sub.3 and SiO.sub.2 are in contact and heated sufficiently, mullite will form as a natural product. This technique requires that the Al.sub.2 O.sub.3 and SiO.sub.2 to diffuse to a common boundary and react chemically. The distance the constitutents diffuse is primarily influenced by the temperature, the time the material is held at temperature, and the particle size of the raw materials.
High purity, submicron mullite powder can be prepared by hydrolytically decomposing a mixture of stoichiometric amounts of aluminum tris-isopropoxide and a silicon tetra-alkoxide in the presence of a weak base or very dilute mineral acid. Mullite has also been prepared by reacting clear, aqueous alumina sol with silicon tetraethoxide.
It is an object of the present invention to provide a novel method for the preparation of mullite.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from the following disclosure of the invention.